Heretofore, ships such as tankers and transport ships include various auxiliary machineries, cargo-handling machines, illumination, air conditioners, and other devices that consume a huge amount of electrical power. In order to supply electricity to these electric systems, the ships include a diesel generator that is a combination of a diesel engine and an electricity generator for generating electrical power when the diesel engine is driven (see, e.g., Patent Literature 1 (hereinafter, PTL1)). It is known that the diesel engine is one of the engines having the highest energy efficiency among internal combustion engines, and the diesel engine emits an exhaust gas containing less carbon dioxide per unit output. Furthermore, the diesel engine can use a low-quality fuel such as heavy oil, which provides an economical advantage.
The exhaust gas from the diesel engine contains carbon dioxide as well as a large amount of substances such as nitrogen oxide, sulfur oxide, and particulate matters. Especially, nitrogen oxide (hereinafter, referred to as NOx) is harmful to a human body and exhibits strong acidity, and is considered to be a cause of acid rain. Machines (such as ships) in which a diesel generator is driven emit a quite large amount of NOx, and therefore are considered to have a great negative effect on the global environment.
As a post-treatment means for greatly purifying NOx, Selective Catalytic Reduction (hereinafter, referred to as SCR), which uses urea as a reducing agent, is commonly used (see, e.g., Patent Literatures 2 and 3 (hereinafter, PTL2 and PTL3)). SCR typically uses a honeycomb-structure NOx catalyst including a support being made of an oxide such as titanium oxide and carrying an active component such as V and/or Cr. When urea water serving as an aqueous reducing agent solution is sprayed to an upstream side of the NOx catalyst, the urea water is thermally decomposed upon heated by an exhaust gas and hydrolyzed, so that ammonia is produced. Ammonia acts on NOx as a reducing agent, and consequently NOx is decomposed into nitrogen and water, which are harmless. PTL2 suggests an exhaust gas purification device including a mixer (exhaust mixer) for mixing an exhaust gas and urea water in order to increase the efficiency of the reduction action of the NOx catalyst.
Considering the global environment, it is necessary to remove NOx in the exhaust gas as much as possible, and it is preferable to regulate NOx emission uniformly both in the high seas and in the territorial seas. Currently, however, along with application of a stricter emission regulation for diesel engines, NOx emission control areas are to be defined in the sea. As described above, the NOx catalyst has a honeycomb structure, and therefore may be clogged with soot and/or particles in the exhaust gas. In addition, the performance of the NOx catalyst may be degraded by sulfur components in the exhaust gas and/or products derived from the sulfur components. In order to extend the service life of the NOx catalyst as long as possible for the purpose of reducing the running cost and achieving compliance with the regulation in the emission control areas in the sea, it is desirable that the NOx catalyst not be exposed to an exhaust gas while the ship is traveling outside the emission control areas.
In order to deal with this, PTL3 proposes an exhaust gas purification device including a purification casing which is disposed in an exhaust gas path of an engine and in which a NOx catalyst is accommodated. In addition, a bypass path causing an exhaust gas to make a detour not to allow the exhaust gas to pass through the NOx catalyst is disposed in the purification casing. In this configuration, an exhaust gas is sent to the NOx catalyst in the purification casing while the ship is traveling in the emission control area, whereas an exhaust gas is sent to the bypass path in the purification casing while the ship is traveling outside the emission control area.